This invention relates to a method and system for control-ing vibrations in borehole equipment comprising a string of tubulars and an associated drive system.
Numerous vibrations may occur in borehole equipment during well drilling or oil production operations. If the equipment includes a rotary drillstring torsional and longitudinal vibrations may be induced by alternating slip-stick motions of the drillstring alongside the borehole wall, by fluctuating bit-rock interaction forces and by pressure pulses in the drilling fluid generated by the mud pumps.
In various situations it is required to damp these vibrations in order to reduce shock loads to the equipment but in some situations it may be required to enhance these loads, for example to create a resonance jar for freeing a stuck drillpipe.
Various concepts are known in the art prior damping or enhancing vibrations in borehole equipment.
U.S. Pat. No. 4,535,972 discloses a system to control vertical movements of a drillstring with the aid of a hydraulic cylinder connected between the traveling block and the top of the drillstring. Although the known system is designed to maintain weight on bit within desired limits it is not operated as a feedback controlled vibration damper.
SPE paper 18049, "Torque Feedback Used to Cure Slip-Stick Motion," presented by G. W. Halsey et al of the Rogaland Research Institute at the Oct., 1988 SPE conference in Houston, describes a system that adapts the value of the speed of the rotary drive to a drilling assembly based on measurement of the torque at the rotary table. The known system is able to perform a rotary speed correction proportional to minus the measured torque.
However, measurement of torque at the rotary table during actual drilling operations is inconvenient and prone to failures, as it involves equipment, such as strain gauges, that is sensitive to vibrations and shockloads.
An object to the present invention is to avoid this drawback of the known system. Another object is to provide a cheap and robust method and system for controlling vibrations in borehole equipment without the need prior complex and wear-prone equipment.